Internet protocol (IP) as developed by the Internet engineering task force (IETF) is implemented on packet-switched networks both for supporting non-real time services such as data transfer services, consulting web pages, or electronic messaging, and for supporting real time or conversational services, such as telephony over IP, video telephony over IP, or indeed video broadcasting over IP.
One method of transporting data streams that present real time constraints is provided by the IETF in Standard RFC 3550. Real time data streams are generally made up of Real Time Protocol (RTP) packets, where an RTP packet comprises digital data derived from analog to digital conversion of signals picked up by sensors or from digital processing of the result of analog-to-digital conversion of signals coming from those sensors. Real time data may also correspond to digital data derived from signals collected by a set of distributed sensors, e.g. when observing road traffic.
A Real Time Control Protocol (RTCP) data stream is associated with a real time data stream. The control data serves in particular to enable equipment that receives a real time data stream to notify one or more pieces of equipment sending the data streams about the quality of the received signal. For this purpose, a distinction is made in RTCP data between Sender Report (SR) control data sent by receiver equipment that also sends an RTP data stream, and Receiver Report (RR) control data sent by receiver-only equipment or by receiver equipment that send a real time data stream, when the number of sender pieces of equipment in a given communication exceeds thirty-one.
According to the Standard RFC 3550, the RTCP control data contains various fields. These RTCP control data fields serve in particular to enable first equipment to notify second equipment sending RTP packets:                of the send instant of the Network Time Protocol (NTP) control data;        of a number of RTP packets received;        of a ratio of RTP packets lost;        of an inter-arrival jitter;        of a first time information field referred to as the Last Sender Report (LSR) field, corresponding to the instant at which the second equipment sends the last RTCP SR data to be received by the first equipment; and        of a second time information field referred to as delay since last sender report (DLSR), corresponding to the time that has elapsed between receiving said most recent RTCP SR packet of control data as sent by the second equipment and the instant at which said RTCP control data is sent.        
These two time information fields enable equipment sending RTCP SR control data to calculate a round trip time for exchanging data with other equipment sending RTCP SR control data. For example, if SSRC_r is receiver equipment that transmits control data relating to sender equipment SSRC_n, the control data including the two time information fields LSR and DLSR, the sender equipment SSRC_n can determine the round trip time rtt between itself and the receiver equipment SSRC_r on the basis of the two fields LSR and DLSR and on the basis of the instant A at which it receives the RTCP SR control data, where:rtt=A−LSR−DLSR
Below, consideration is given to the situation in which a plurality of data sources send respective real time data streams made up of RTP packets in broadcast mode or multicast mode. A first source of the plurality sends real time data and the other sources receive the stream of data. A plurality of RTCP control data streams need to be established, where such a stream is exchanged between the first source and each of the other sources. For each of the other sources, the first source may determine a transmission time and each may use the determined transmission times in order to aggregate the real time data streams from other sources with its own real time data stream. The number of RTCP control data streams increases as a function of the number of RTP real time data sources. For any one broadcast source of real time data in a large-sized multicast broadcast group, managing the RTCP control data streams becomes extremely complex and consumes large amounts of processor and memory resources.